The objective of this study is to understand the winter biology of Culex tarsalis, an important pest and disease vector (western equine encephalitis and St. Louis encephalitis). Project goals are to (1) characterize behavioral and physiological changes in autumn populations of C. tarsalis related to preparation for hibernation, (2) establish the environmental factors inducing these changes, (3) establish the relationship between winter environmental conditions and mosquito survival potential, (4) evaluate the winter survival potential for blood-fed and non-bloodfed hibernating female mosquitoes, (5) estimate the frequency of bloodfeeding in prehibernating female mosquitoes and (6) examine geographic variations in winter biology of C. tarsalis. This research should contribute to our knowledge of the epidemiology of North American mosquito-borne virus diseases of man -- specifically how these viruses survive interepidemic and interepizootic periods.